1. Field of Invention
The invention relates to the field of semiconductors. More particularly, the invention is directed to group III-V nitride semiconductor films usable in blue light emitting devices.
2. Description of Related Art
The light-emitting diode is the basic component for electronic lighting technology. A light-emitting diode is a relatively simple semiconductor device which emits light when an electric current passes through a p-n junction of the light-emitting diode. As shown in FIG. 1, a light-emitting diode 100 includes a p-type 110 semiconductor material adjacent to an n-type 120 semiconductor material, i.e., a p-n junction, characterized by a bandgap energy E.sub.g 130. The bandgap energy 130 is the minimum energy required to excite an electron 160 from a valence band 140 to a conduction band 150, where the electron 160 becomes mobile. Likewise, the bandgap energy 130 also determines the energy of a photon produced when the electron 160 in the conduction band, i.e., a conduction electron, recombines with a hole 170, i.e., an unoccupied electronic state, in the valence band 140. When forward current passes through the diode 100, the electrons 160 in the conduction band 150 flow across the junction from the n-type material 120, while the holes 170 from the valence band 140 flow from the p-type material 120. As a result, a significant number of the electrons 160 and the holes 170 recombine in the p-n junction, emitting light with an energy E.sub.photon =E.sub.g. These semiconductor devices, comprising a p-n junction, in a single material, and are referred to as homojunction diodes.
In order to obtain more efficient LEDs and laser diodes, in particular, lasers that operate at room temperature, it is necessary to use multiple layers in the semiconductor structure. These devices are called heterojunction or heterostructure LEDs or lasers.
The wavelength, and thus the color of light emitted by an LED or laser diode, depends on the bandgap energy E.sub.g. LEDs or laser diodes that emit light in the red-to-yellow spectrum have been available since the 1970's. There has been great difficulty, however, in developing efficient LEDs that emit light at shorter wavelengths. Extending LED light sources into the short-wavelength region of the spectrum, the region extending from green to violet, is desirable because LEDs can then be used to produce light in all three primary colors, i.e., red, green, and blue. Shorter-wavelength laser diodes will likewise enable full-color projection displays; and they will also permit the projection of coherent radiation to focus laser light into smaller spots. That is, in the optical diffraction limit, the size of the focused spot is proportional to the wavelength of the light. Reducing the wavelength of the emitted light allows optical information to be stored at higher densities and read out more rapidly.
FIG. 2 shows a conventional LED structure 200 in which an InGaN active layer 230 is formed over a group III-V nitride layer 220. Specifically, as shown in FIG. 2, the conventional LED 200 includes a substrate 205, which may, for example, be formed of sapphire or silicon carbide. A buffer layer 210 is formed on the substrate 205. The group III-V nitride layer 220 is then formed on the buffer layer 210. The group III-V nitride layer 220 is typically GaN. The InGaN active layer 230 is formed on the group III-V nitride layer 220. A second group III-V nitride layer 240 is then formed on the InGaN active layer 230. A third group III-V nitride layer 250 is formed on the second group III-V layer 240. The first group III-V nitride layer 220 is n-type doped. The second and third group III-V nitride layers 240 and 250 are p-type doped. A p-electrode 260 is formed on the third group III-V nitride layer 250. An n-electrode 270 is formed on the first group III-V nitride layer 220.